METHODE ET APPAREIL POUR CALCULER UN RECIPROQUE

METHOD AND APPARATUS FOR CALCULATING A RECIPROCAL

Abrégé anglais

A method and apparatus for the calculation of the reciprocal of a normalized
mantissa M for a floating-point input number D . A formula for determining the
minimum size for the look-up table in accordance with the required precision
is provided,
as well as formulas for calculating look-up table entries. The look-up table
stores the
initiation approximations and the correction coefficients, which are addressed
by the
corresponding number of the mantissa's most significant bits and used to
obtain the initial
approximation of the reciprocal by means of linear interpolation requiring one
subtraction
operation and one multiplication operation. The result of the linear
interpolation may be
fed to a Newton-Raphson iteration device requiring, for each iteration, two
multiplication
operations and one two's complement operation, thereby doubling the precision
of the
reciprocal.

Revendications

WE CLAIM:
1. A method for generating an output signal representing an output value
approximating a reciprocal of input value D having a normalized mantissa M
(where
1 .ltoreq. M < 2 ) represented by an input signal, the input signal comprising
a set of N 0 most
significant bits and the output signal approximating the reciprocal with a
desired precision
.epsilon. = 2-N where N .ltoreq. N o , comprising the steps of:
a. for a set of P most significant bits of the input signal, generating a
number n of
entries in a plurality of lookup tables where n = 2P , including the sub steps
of:
i. generating a set of input entries y i comprising a set of N significant
bits
in a first lookup table, where i = 0,..., n - 1; and
ii. generating a set of input entries K i comprising a set of (N - P)
significant
bits in a second lookup table, where i = 0,..., n - 1;
b. finding the entries y i and K i in the lookup tables corresponding to the
set of P
most significant bits of the input signal;
c. multiplying K i by a signal comprising a set of (N - P) significant bits
following
the set of P most significant bits of the input signal; and
d. subtracting a set of (N - P) most significant bits from the set of N
significant
bits of the entry y i.
2. The method of claim 1 wherein the step of generating n entries in the
lookup
tables comprises the sub steps of:
iii. calculating the minimum number l of lookup table entries necessary to
obtain a precision higher than the desired precision, where
<IMGS>
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iv. finding a required minimum number n of lookup table entries for n = 2 P,
where 2 P-1 < l and 2 P .gtoreq. l.
3. The method of claim 1 wherein the step of generating a set of input entries
in the
first lookup table comprises the sub steps of:
A. calculating <IMG> where i = 0,..., n - 1, x0 = 1, and
<IMG> ;
B. finding entries .gamma.i comprising a set of N significant bits and
approximating a mantissa of ~i for i = 0,..., n - 1.
4. The method of claim 1 wherein the step of generating a set of input entries
in the
second lookup table comprises the sub steps of:
A. calculating <IMG> where i = 0, ..., n - 1, x0 = 1, and
<IMG>
B. finding entries K i comprising a set of (N - P) significant bits and
approximating integer parts of ~i for i = 0,..., n - 1.
5. An apparatus for calculating an inverse I having a precision .epsilon. = 2-
N of an input
value D with normalized mantissa M (where 1 .ltoreq. M < 2) comprising a set
of N0 most
significant bits where N0 .gtoreq. N, the apparatus comprising
at least one processor,
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a first memory forming a lookup table addressed as a function of P most
significant bits of the mantissa M and having an output I0 comprising a set of
N
significant bits;
a second memory forming a lookup table addressed as a function of P most
significant bits of the mantissa M and having an output K comprising a set of
(N - P) significant bits;
a multiplier of size (N - P) × (N - P) having two inputs of a set of (N -
P)
significant bits following the set of P most significant bits of the mantissa
M
and of the output K, and an output MU comprising a set of (N - P) × (N -
P)
significant bits; and
an adder/subtracter having an output I and having two inputs connected to
respectively receive the output I0 and the set of (N - P) most significant
bits of
the output MU.
6. The apparatus of claim 5 wherein the first and second memories are combined
into a storage device which stores both I0 and K and is addressed as a
function of P
most significant bits of the mantissa M.
7. The apparatus of claim 5 further comprising a device for performing a
programmed Newton-Raphson iteration based on I.
8. The apparatus of claim 5 wherein the first memory comprises a read only
memory
(ROM).
9. The apparatus of claim 5 wherein the second memory comprises a read only
memory (ROM).
10. The apparatus of claim 6 wherein the storage device comprises at least one
read
only memory (ROM).
11. A digital signal processing device comprising the apparatus of claim 5.
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Description

CA 02329104 2004-08-11
METHOD AND APPARATUS FOR CALCULATING A RECIPROCAL
Field of Invention
This invention relates to signal processing. In particular, this invention
relates to a
method and apparatus for calculating the reciprocal or inverse of a number.
Background of the Invention
Calculating the reciprocal plays an important role in the division operation,
especially with floating-point decimal numbers. By using a reciprocal, the
result of the
division of two numbers can be obtained by multiplying the dividend by the
inverse of the
divisor. This division method can be used to increase the speed of processing
complex
calculations in digital processing devices such as computers and in
application-specific
integrated circuits such as digital signal processing (DSP) processors.
According to IEEE Standard for Binary Floating-Point Arithmetic 0754P-1985,
floating-point standard numbers in floating-point formats are packed within 32
bits with a
significand (mantissa) 24 bits long in single precision, or packed within 64
bits with a
significand 53 bits long in double precision.
Several interpolation and iteration methods are widely used by developers for
calculating reciprocals, including direct approximation, linear interpolation,
square
interpolation, cubic interpolation, and so on.
In the direct approximation method of obtaining reciprocal of a number, all
possible mantissas for reciprocals are stored in a ROM table. Using this
method the result
can be obtained quickly, but this method requires an extremely large memory
capacity.
For example, to obtain a reciprocal according to the IEEE standard 754 single
precision
floating-point format requires 223 x 23 = 184 Mbits of memory.
The linear interpolation method is based on the theorem of a mean value from
calculus, and can be summarized for the calculation of reciprocal as follows:
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1 - 1 - ~1z ( x - xo ) ( 1 )
x xo
where ~ E [ xo , x] and x >_ xo .
It is also possible to use square interpolation, cubic interpolation and other
interpolation methods to obtain a reciprocal with the required precision.
However, all of
these methods require additional multiplication operations, and require
additional
memory to store the correction coefficients. The main disadvantage of
interpolation
methods is that as the desired precision increases, the amount of memory
required to store
the necessary data also increases.
In digital computers the Newton-Raphson iteration method is widely used for
calculating reciprocals. This method gives the solution of the equation
(2)
.f(z)=0
based on employing the recurrent formula
zr+W za - f ~ ~ ~
z;
The values z; obtained after iteration i are quadratically converging toward z
, so the
corresponding errors E after iteration i and iteration i + 1 relate by the
expression:
E (z~+~) ~EZ(zr)
Employing the Newton-Raphson method for calculating reciprocal x = 1
a
produces the following expression:
xr+~ = xr * (2 - a * xt ) (S)
As can be seen from equation (5), every iterative step of this method involves
two
multiplication operations performed in sequence, and one '2's complement'
operation.
The precision of a reciprocal thus doubles after each iterative step. The
disadvantage of
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CA 02329104 2000-12-20

the Newton-Raphson iteration method by itself is that it can require multiple
iteration
steps to obtain a reciprocal with the required precision.
To overcome the above disadvantages methods have evolved to use some type of
interpolation method to obtain the initial approximation of a reciprocal, and
then employ
an iteration method based on this approximation. As an example, it has been
proposed to
use inverse tables to obtain the initial values for consecutive iterations.
Summary of the Invention
The present invention provides a method and apparatus for dividing a value
which
can deliver the inverse (reciprocal) of a number quickly and with a high
precision.
According to the method of the invention, linear interpolation is employed to
obtain an approximation of the reciprocal of a number. This approximation may
then be
used as an input value for Newton-Raphson iterations to calculate a reciprocal
with high
precision.
Unlike prior art methods, the method of the invention provides a formula for
calculating a minimum number of entries in a look-up table to obtain the
approximation
of a reciprocal with required precision. The method of the invention also
provides
formulas for calculating initial approximations and correction coefficients
for composing
entries in look-up tables. An apparatus for implementing the method of the
invention
comprises a look-up table memory for storing these values, an integer
multiplier, and a
subtracter.
The present invention thus provides a method for generating an output signal
representing an output value approximating a reciprocal of input value D
having a
normalized mantissa M (where 1 <_ M < 2 ) represented by an input signal, the
input
signal comprising a set of No most significant bits and the output signal
approximating
the reciprocal with a desired precision E = 2-" where N <_ No , comprising the
steps o~
a. for a set of P most significant bits of the input signal, generating a
number n of entries
in a plurality of lookup tables wheren = 2p, including the sub steps of: i.
generating a set
of input entries y; comprising a set of N significant bits in a first lookup
table, where i =
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CA 02329104 2000-12-20

0,..., n - 1; and ii. generating a set of input entries K; comprising a set of
( N - P )
significant bits in a second lookup table, where i = 0,..., n - 1; b. finding
the entries y; and
K; in the lookup tables corresponding to the set of P most significant bits of
the input
signal; c. multiplying K; by a signal comprising a set of ( N - P )
significant bits
following the set of P most significant bits of the input signal; and d.
subtracting the set
of ( N - P ) most significant bits from the set of N significant bits of the
entry y; .
In further aspects of the method of the invention: the step of generating n
entries
in the lookup tables comprises the sub steps o~ iii. calculating the minimum
number l of
lookup table entries necessary to obtain a precision higher than the desired
precision,
where
2l+1 _ l <s and 2l-1 _ l 1 ~E
2l+2 l+1 2l
and iv. finding a required minimum number n of lookup table entries for n =
2P, where
2p-' < l and 2P >_ l ; the step of generating a set of input entries in the
first lookup table
comprises the sub steps o~ A. calculating
1
x~ (x~ + -) +
n 2n
y; = 1
x~ (x; + -)
n
where i = 0,..., n - l, xo =1, and x;+, = x; + 1 , and B. finding entries yi
comprising a set
n
of N significant bits and approximating a mantissa of y; for i = 0,..., n - 1;
and/or the
step of generating a set of input entries in the second lookup table comprises
the sub steps
o~ calculating
1
K; = where i = 0,..., n - 1, xo =1, and x;+, = x; +
1 n
x; (x; + -)
n
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CA 02329104 2000-12-20

and finding entries K~ comprising a set of ( N - P ) significant bits and
approximating
integer parts of K; for i = 0,..., n - 1.
The present invention further provides an apparatus for calculating an inverse
I
having a precision s = 2-" of an input value D with normalized mantissa M
(where
1 <- M < 2 ) comprising a set of No most significant bits where No >- N , the
apparatus
comprising at least one processor, a first memory forming a lookup table
addressed as a
function of P most significant bits of the mantissa M and having an output
to comprising a set of N significant bits; a second memory forming a lookup
table
addressed as a function of P most significant bits of the mantissa M and
having an
output K comprising a set of ( N - P ) significant bits; a multiplier of
size (N - P) x (N - P) having two inputs of a set of ( N - P ) significant
bits following
the set of P most significant bits of the mantissa M and of the output K, and
an output
MU comprising a set of (N - P) x (N - P) significant bits; and an
adder/subtracter
having an output I and having two inputs connected to respectively receive the
output
to and the set of ( N - P ) most significant bits of the output MU .
In further aspects of the apparatus of the invention: the first and second
memories
are combined into a storage device which stores both to and K and is addressed
as a
function of P most significant bits of the mantissa M ; the apparatus further
comprises a
device for performing a programmed Newton-Raphson iteration based on I ; the
first
memory comprises a read only memory (ROM); the second memory comprises a read
only memory (ROM); the storage device comprises at least one read only memory
(ROM); and or the apparatus is included in a digital signal processing device.
Brief Description of the Drawings
In drawings which illustrate by way of example only a preferred embodiment of
the invention,
Figure 1 is a graph showing the linear interpolation method as used in the
present
invention,
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CA 02329104 2000-12-20

Figure 2 is a block diagram showing a linear interpolation apparatus according
to
the invention for obtaining N-bit precision for the mantissa of a reciprocal,
and
Figure 3 is a block diagram showing a Newton-Raphson iteration apparatus for
obtaining 2N-bit precision for the mantissa of a reciprocal.
Detailed Description of the Invention
The invention is applicable to calculations utilizing the mantissa M of a
binary
number D in the floating-point format. The mantissa M of the input number is
assumed
to have been preliminarily normalized, i.e. being equal to or more than one
and less than
two, such that 1 <_ M < 2 .
Figure 1 illustrates a preferred embodiment of the linear interpolation method
used in the method of the invention to approximate the reciprocal of a number.
Relative to
the direct approximation method, linear interpolation significantly reduces
the number of
entries required to be stored in a look-up table. To achieve an N-bit
precision E = 2-N for
the mantissa M of reciprocal while employing the direct approximation method,
the look-
up table would require 2 N -1 equally spaced entries; whereas to obtain the
same N-bit
precision E = 2-N for the reciprocal's mantissa using the linear interpolation
method, it is
sufficient to have 2p entries in the look-up table where P <_ N . Each entry
is distinct
from the previous entry by 2-P , so P most significant bits (MSB) of the
mantissa M
constitute the look-up table address.
Values x; and x;+, in Figure 1 represent two consecutive entries in the look-
up
table. Value y; represents the approximation of the value 1 that is stored in
the look-up
xr
table. Value ( x - x; ) is represented by mantissa's bits next after its first
P MSB.
The approximation of a required reciprocal is expressed by the following
formula:
Y = Y. - kr(x - xr) (6)
where k; is correction coefficient, and xi <_ x < x;+, .
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CA 02329104 2000-12-20

To obtain a reciprocal's mantissa M with N-bit precision, at least ( N - P )
bits
after P MSB of the mantissa M must be matched with at least ( N - P )
significant bits of
correction coefficient k; shown in Figure 1. These coefficients are stored in
the look-up
table in integer form K; = 2N-P X k~ . Therefore, to obtain the reciprocal's
mantissa M
with N-bit precision involves the steps of multiplying the ( N - P ) bits of
the mantissa M
by the ( N - P ) bits of coefficient K; in the integer multiplier of size (N -
P) x (N - P) ,
dividing the result by 2 N-P , and then subtracting the quotient from y; .
The N bits of mantissa M for y; and the ( N - P ) bits for integer K; are
stored in
the look-up table. Values y; and K; are calculated according to the following
formulas
with N and ( N - P ) precision, respectively:
1
x' (x' + n) + 2n
y~ = (~)
x; (x~ + 1 )
n
2 N_P
Ka - (8)
x~ (x; + 1 )
n
where n is the number of entries in the look-up table.
The maximum error s of the linear interpolation method for finding the
reciprocal
of a mantissa M, normalized to lie between 1 and 2, depends on n and is
expressed by the
following formula:
CA 02329104 2000-12-20

_ 2n+1 n ( )
2n+2 n+1
The necessary number n of entries in the look-up table can be determined from
formula (9) where the required maximum error s is known.
For example, for n = 64 the maximum error E of a linear interpolation method
for obtaining a reciprocal is equal to s ~ 2.98 * 10-5 > 2-'6 , while for n =
128 the
maximum error according to the linear interpolation method is ~ ~ 7.54 * 10-6
< 2-'6 .
Figure 2 illustrates an apparatus 10 for calculating reciprocals according to
the
invention, which implements the method as described above. The P MSB of
mantissa
M of the input number D form the address line of the ROM 12 that has 2P
entries. The
ROM 12 preferably stores N bits for the inverse y; of a mantissa's P MSB
(although it
is sufficient to store ( N -1 ) bits since the leading bit of the inverse is
always '0') and
( N - P ) bits of the correction coefficient K; , to perform the linear
interpolation.
The ( N - P ) bits of correction coefficients K are provided to one input of
integer
multiplier 14 having a size ( N - P ) x ( N - P ). Provided to another input
of the multiplier
14 is the ( N - P ) bits value next after the P MSB of the input mantissa M .
The
( N - P ) MSB MU of the product obtained by multiplying ( N - P ) x ( N - P )
bits long
are provided to an input of integer subtracter 16 having a size N . The P MSB
of the
input MU are all '0', and the ( N - P ) least significant bits (LSB) of the
product are
discarded. Provided to another input of the subtracter 16 are N bits of the
reciprocal's
approximation y; from the ROM 12 (shown as Io in Figure 2). The result of
subtraction
forms the N bits output (shown as I in Figure 2) of the apparatus 10.
Note if P MSB of the input mantissa M are equal to ' 1' (i.e. the most signif
cant
bit is ' 1' while other ( P -1 ) bits are '0'), then the output I can be
represented as 1's
complement of ( N - P ) bits of MU , which simplifies the calculations.
_g_
CA 02329104 2000-12-20

Fig. 3 illustrates an apparatus 20 for performing Newton-Raphson iterations of
the
result of linear interpolation, to increase the precision of the result from N-
bit to 2N-bit
precision. The N bits of output I from the interpolation apparatus 10 are
provided to an
input of integer multiplier 22 having a size N x 2N . Provided to another
input of the
multiplier 22 is 2N bits of output from the multiplexer 24. Multiplexer 24
alternately
selects 2N MSB of the input mantissa M (padded with appended '0's if
necessary) and
2N bits of the output of a 2's complement device 26.
Multiplier 22 produces a result 3N bits long. The N least significant sets of
the
product of this multiplication is discarded. The 2N most significant bits MUl
of the
product are provided to the 2's complement device 26. The output of the 2's
complement
device 26 2N bits long is provided to the multiplexer 24. On the second pass
through the
multiplier 22 the 2N MSB of the product of the multiplication form the 2N bits
output
of the apparatus 20 (shown as an I1 in Figure 3).
While a preferred embodiment of the present invention has been shown and
described by way of example only, it will be apparent to those skilled in the
art that
changes and modifications may be made without departing from the scope of the
invention, as set out in the appended claims.
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CA 02329104 2000-12-20

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